This invention relates to alarms and signaling apparatus for generating an audible signal in response to an applied electrical signal. More specifically, this invention relates to circuit arrangements for generating an acoustic signal wherein the arrangement is suitable for use as a replacement for (or alternative to) the electromagnetic ringers or bells that are utilized in communications equipment such as conventional telephone sets and other telephone station apparatus.
Numerous arrangements that include electronic circuitry and an electroacoustic transducer have been proposed as an alternative or replacement for conventional electromagnetic ringers of telephone sets and other telephone station apparatus. Nevertheless, the prior art has not provided an arrangement which exhibits overall performance characterics equal to, or in many cases, even approaching those of the various readily available electromagnetic ringing devices. Thus, prior to the present invention, the use of electronic ringers or, as they are sometimes identified, tone ringers has been primarily limited to special purpose station apparatus wherein the desirability or necessity of one or more features of an electronic ringer offsets the resulting compromises relative to other characteristics, including various performance parameters and factors such as cost.
In general, the problems associated with developing an electronic ringer that is suited for widespread usage within various types of telephone sets and other apparatus have not been related to physical characteristics such as size and weight, nor has a serious problem been encountered relative to device reliablity. In fact, the inherent reliability and long service life of properly designed electronic circuits has been one of the primary motivations in attempting to replace conventional electromagnetic ringers with electronic ringing apparatus. Further, although presently available electromagnetic ringers are of relatively low mass and are small in size, state of the art circuit design techniques, including the use of integrated circuit technology, permit realization of electronic ringers of suitable size and weight.
Although providing a highly discernible audible signal of acceptable level and tonal quality while simultaneously achieving the desired size, weight and reliability places additional constraints on the designer of electronic ringer apparatus, the primary problems encountered relate to configuring the electronic ringer so that it is compatible with various subscriber loops that are utilized in present day telephone systems. In this regard, the primary requirement is that the ringer device must detect and be responsive to the ringing signal that is supplied by the telephone central office serving that subscriber. As is known in the art, the ringing signal conventionally employed is a relatively low frequency, high amplitude alternating current that is intermittently or cyclically coupled to the station equipment being signalled via the subscriber loop. For example, the ringing cycle of a typical telephone system may include approximately a two-second interval of alternating current having an amplitude of approximately 80-90 volts rms and a nominal frequency of 20 hertz, followed by an interval of approximately four seconds in which no alternating current signal is provided, with the ringing cycle being repeated until the called party answers or the calling party terminates the attempted call.
One problem encountered in attempting to utilize electronic circuitry to detect the ringing signal is that the signal must be detected in the presence of a wide variety of numerous other signals that are intentionally and unintentionally coupled to the telepone line even though the amplitude of the ringing signal varies over a wide range and various frequency ringing signals are employed. For example, variations in the signal level supplied by conventional ringing generators and variations in loop impedance can result in ringing signals which range in amplitude between 40 and 150 volts rms (at the station apparatus), at a frequency of between approximately 16 and 66 hertz, depending on the particular telephone installation of concern.
As is known to those skilled in the art, various transient signals that are encountered on the telephone line exhibit a signal level comparable to the above-mentioned range of riging signal. In fact, such transient signals can include several repetitive pulses at a repetition rate similar to the signal frequency of conventional ringing generators. For example, conventional telephone dial-pulsing apparatus utilizes a nominal pulse repetition rate of approximately ten pulses per second with a nominal 60% make-break ratio. Dial-pulsing, as well as numerous other supervisory and switching operations that are performed in a conventional telephone system, can cause a signal to be coupled to the associated station apparatus at an amplitude that exceeds the telephone system DC supply voltage (normally 48 volts), especially when the supervisory or switching operation being performed operates central office relays or other devices that establish a collapsing electromagnetic field. These high-level, transient signals are undesirable and often cause a condition referred to as "bell tap" wherein a conventional electromagnetic ringer is momentarily energized and emits a ringing signal of less than normal time duration.
Although bell tap has been somewhat of a problem with respect to electromagnetic ringers, the analogous problem of designing an electronic ringer that, in effect, discriminates between actual ringing signals and somewhat similar high voltage transients or other signals appearing on the telephone line has been far more troublesome. In particular, the designer of an electromagnetic ringer can control or adjust the mass and other physical constants associated with conventional electromagnetic ringers so that the ringer, in effect, acts as a filter network that does not allow the striker or hammer of the ringer to contact the bell surface unless the electrical energy supplied to the ringer coil substantially corresponds to that which would be supplied by an actual ringing signal. Although various attempts have been made to design a detector circuit for an electronic ringer that is an electrical analog of an electromagnetic ringer, an arrangement suited for use in actual practice has not been heretofore realized.
Not only must a satisfactory electronic ringer in effect discriminate between actual ringing signals and other signals appearing on the telephone line, but such a ringer must not adversely affect the transmission characteristics of the telephone line with which it operates and must not interfere with the various DC supervisory functions of conventional telephone systems. For example, to maintain proper signal balance and low noise when an electronic ringer is utilized in a divided ringing situation, i.e., connected between earth ground and the tip or ring conductor of a two-wire telephone pair, the electronic ringer must exhibit a relatively high impedance. Further, in many installations, an electronic ringer must be AC coupled to the telephone line so that a DC signal does not flow through the ringer that would be detected by conventional central offices as either an indication of a station set off-hook condition or excessive signal leakage within that particular telephone loop. In addition, since conventional telephone systems utilize a power source that is independent of commercial power distribution systems in order to remain operational during periods of commercial power failure, it is desirable and even necessary for an electronic ringer to derive all of its operating power from the telephone line. Combining the requirements of AC coupling, high circuit impedance and the need for powering the ringer from the ringing signal with the requirement that the acoustic output of such a device be at least a predetermined level, it can be recognized that a satisfactory electronic ringer must exhibit a high level of circuit efficiency that is not easily attained with conventional circuit design techniques.
In addition to the above-discussed design criteria, it is desirable that an electronic ringer circuit to be readily adaptable for use in as many types of station apparatus and various installations of such apparatus as possible. In this regard, it is especially desirable that an electronic ringer be easily adaptable for use in multiparty subscriber loops such as the previously-mentioned divided ringing installations. In addition, a need often arises for a ringer that can be utilized in multiline and intercom applications such as those presented by relatively small key telephone systems. Other, more specialized situations can also be encountered such as providing a variable or adjustable ringer capable of generating a distinctive signal when operated with station apparatus at a location that includes multiple installations of various types of station equipment. If several such situations can be accommodated by a single ringer design or a design having a number of common circuit stages, both manufacture and servicing of the electronic ringer are facilitated and become more economical.
Accordingly, it is an object of this invention to provide electronic ringer apparatus operable with a variety of conventional telephone system arrangements.
It is another object of this invention to provide an efficient electronic ringer that includes circuitry for distinguishing between actual telephone ringing signals and various other electrical signals that may be present on a telephone line to thereby eliminate or substantially reduce the occurrence of the condition known as bell tap.
It is yet another object of this invention to provide an electronic ringer that meets each of the above set forth objectives and, additionally, provides a pleasant, highly discernible audible ringing signal.
Still further it is an object of this invention to provide an electronic ringer that is readily adaptable for use on multiparty subscriber loops such as divided ringing installations and is readily adaptable for use in various other telephone station installations.